1. Field of the Invention
The present invention relates to a method of driving a plasma display panel used as a thin display device having a large screen and light weight.
2. Description of Related Art
An alternating current surface-discharging panel representing a plasma display panel (hereinafter abbreviated as a panel) has a plurality of discharge cells formed between facing front panel and rear panel. In the front panel, a plurality of display electrodes, each formed of a pair of scan electrode and sustain electrode, are formed on a front glass substrate in parallel with each other. A dielectric layer and a protective layer are formed to cover these display electrodes. On the other hand, in the rear panel, a plurality of data electrodes is formed in parallel with each other on a rear glass substrate. A dielectric layer is formed on the data electrodes to cover them. Further, a plurality of barrier ribs are formed on the dielectric layer in parallel with the data electrodes. Phosphor layers are formed on the surface of the dielectric layer and the side faces of the barrier ribs. Then, the front panel and the rear panel are arranged to face each other and sealed together so that the display electrodes and data electrodes intersect with each other, and a discharge gas is filled into an internal discharge space formed therebetween. A discharge cell is formed at a part where a display electrode is faced with a corresponding data electrode. In a panel structured as above, ultraviolet light is generated by gas discharge in each discharge cell. This ultraviolet light excites respective phosphors to emit R, G, or B color, for color display.
A general method of driving a panel is a so-called sub-field method: one field period is divided into a plurality of sub-fields and combination of light-emitting sub-fields provides gradation images for display. Among such sub-field methods, a novel driving method of minimizing light emission unrelated to gradation representation to improve a contrast ratio is disclosed in Japanese Patent Unexamined Publication No. 2000-242224.
FIG. 8 shows an example of driving waveforms of a conventional plasma display panel with an improved contrast ratio. These driving waveforms are described hereinafter. One field period is composed of n sub-fields, each having an initializing period, writing period, and sustaining period. The sub-fields are abbreviated as a first SF, second SF, and so on to an n-th sub-field. As described below, in sub-fields except the first SF among these n sub-fields, initializing operation is performed only on discharge cells that have been lit during the sustaining period of the previous sub-field.
In the former half of the initializing period of the first SF, application of a gradually-increasing ramp voltage to scan electrodes causes weak discharge so that wall electric charge necessary for writing operation is provided on each electrode. At this time, in order to optimize the wall electric charge afterwards, excessive wall electric charge is provided. In the following latter half of the initializing period, application of a gradually-decreasing ramp voltage to the scan electrodes causes weak discharge again, to weaken the wall electric charge excessively stored on each electrode and adjust the wall electric charge to a value appropriate for each discharge cell.
In the writing period of the first SF, writing discharge is caused in discharge cells to be lit. In the sustaining period of the first SF, sustain pulses are applied to scan electrodes and sustain electrodes to cause sustaining discharge in the discharge cells in which writing discharge has occurred. Thus, the phosphors of the corresponding discharge cells emit light for image display.
In the following initializing period of the second SF, the same driving waveforms as the latter half of the initializing period of the first SF, i.e. a gradually-decreasing ramp voltage, is applied to the scan electrodes. This is because the wall charge necessary for writing operation is provided at the time of sustaining charge and thus the former half of the initializing period need not be provided independently. Therefore, weak discharge occurs in the discharge cells in which sustaining discharge has occurred in the first SF, to weaken the wall discharge excessively stored on each electrode and adjust the wall discharge to a value appropriate for each discharge cell. In discharge cells in which no sustaining discharge has occurred, the wall charge at the time of completion of the initializing period of the first SF is maintained. Thus, discharge does not occur.
As described above, the initializing operation in the first SF is an all-cell initializing operation in which all the cells are discharged. The initializing operation in the second SF or after has a selective initializing operation in which only discharge cells subjected to sustaining discharge are initialized. For this reason, light emission unrelated to display is weak discharge occurring in the initializing operation of the first SF only. Thus, images with high contrast can be displayed.
However, in spite of display of images with high contrast, the above driving method has a problem of increasing voltage applied to the data electrodes in order to ensure the wiring discharge.
The present invention addresses the above problem and aims to provide a method of driving a plasma display panel capable of displaying images with high contrast without increasing the voltages applied to the data electrodes.